1. Field of the Invention
This invention relates to a semiconductor pressure sensor which is substantially impervious to electrical interference.
2. Description of the Related Art
A semiconductor pressure sensor for measuring a pressure by detecting a variation of resistance value caused by a deformation of a strain gauge is well known, and usually comprises a diaphragm made of a single crystal silicon substrate and having a thin portion. The variations of resistance are measured by detecting the value of a pressure applied to the diaphragm, utilizing the concept that the deformation of the strain gauge varies in accordance with a mechanical strain force applied thereto by a piezo-resistance element.
Japanese Unexamined Patent Publication 61-239675 discloses a semiconductor pressure sensor, as shown in FIG. 22, which comprises a semiconductor substrate 800 on which a piezoresistance layer 802 is formed, a semiconductor substrate layer 810 for supporting the semiconductor pressure sensor, and a buried isolation layer 808 provided between the semiconductor substrate 800 and the semiconductor substrate layer 810. This sensor is characterized in that a bimetallic movement caused by contact between the diaphragm having a thin thickness and another isolation layer formed on the surface thereof, is suppressed.
As shown in FIG. 22, a conductive metal layer 804 and an isolation layer 806 are further provided in the semiconductor pressure sensor.
In the semiconductor pressure sensor as shown in FIG. 22, however, several drawbacks have been found, for example, the semiconductor substrate 800 and the semiconductor substrate layer 810 are inter conductive when they come into contact with each other during a dicing operation for separating a wafer into respective chips, because the thickness of the buried isolation layer 808 provided therebetween is very thin, e.g., about 0.5 to 2 .mu.m, or when water or dust is attached to an outer peripheral side surface of the buried isolation layer 808 or in the vicinity thereof, even after the semiconductor pressure sensor is produced. Further, when an electrical voltage exceeding a surface discharge breakdown voltages is applied, due to noise or the like, between the semiconductor substrate 800 and the semiconductor substrate layer 810, an external potential will be introduced into the semiconductor substrate, leading to a malfunction or variations in the output thereof due to a mutual interaction between the substrate and the circuitry of a chip pressure sensor on which a drive source of the semiconductor pressure sensor and various circuits are integrated.